how to change fax api from address on coversheet

You can set your custom coverpage where you can customize the senders and receivers info:

ref: https://support.ringcentral.ca/articles/en_US/RC_Knowledge_Article/Fax-Create-a-Custom-Fax-Cover-Page-RingCentral-Phone

In case of API, "If you want to use your own custom cover page, disable the system cover page by setting coverIndex to 0 and then adding your own cover page as the first page of your fax message. "

ref: https://ringcentral-faq.readthedocs.io/en/latest/fax/

Also you can check this: https://community.ringcentral.com/questions/987/online-fax-customize-cover-sheet.html

Hope these helps